Integrated gasification combined cycle (IGCC) plants are power plants wherein coal and other carbon-based fuels are submitted to a combustion process and turned into gases. Carbon dioxide (CO2) emissions produced as a result of the combustion process has recently become a major environmental concern; thus, different methods have been developed in order to remove CO2 from gas streams produced in IGCC plants.
It is known, for example, to remove CO2 from gas streams by chemical absorption with amines (typically monoethanolamine), if the gas stream is at low pressure, or by physical absorption if the mixture is at high pressure.
CASIMIRO, T., et al. Phase behavior studies of a perfluoropolyether in high-pressure carbon dioxide. Fluid Phase Equilibria. 2004, no. 224, p. 257-261. reports on vapor-liquid equilibrium measurements for the binary system CO2-Krytox® 157 FSL (per)fluoropolyether (PFPE), a carboxy-terminated branched PFPE comprising units of formula —CF(CF3)CF2O—. No data or information is reported in this article concerning the solubility of other gases in the PFPE and no mention or suggestion is given on the possible use of other PFPEs for the dissolution/extraction of CO2.
MILLER, Mathew B., et al. CO2-philic Oligormer as Novel Solvents for CO2 Absorption. Energy&Fuels. 2010, vol. 24, p. 6214-6219. teaches the use of certain oligomers as solvents for CO2 absorption in an integrated gasification combined cycle plant. This document teaches that the desirable properties of such solvents are selectivity of CO2 over H2 and water, low viscosity, low vapor pressure, low cost and minimal environmental, health and safety impact. According to this document, polypropylene glycol dimethylether (PPGDME) and polydimethyl siloxane (PDMS) revealed the most promising solvents. A branched PFPE or PFPE glycol of formula CF3CF2[OCF2CF(CF3)]nF was also tested; however, on page 6216, right-hand column, lines 17-20, it is stated that the tested PFPE gave poor performance in a test wherein the solubility of CO2 was measured. This prior art does not report data on the absorption selectivity CO2 over N2.
MILLER, Matthew B., et al. Solubility of CO2 in CO2-philic oligomers; COSMOtherm predictions and experimental results. Fluid phase equilibria. 2009, vol. 287, no. 1, p. 26-36. reports on a study aimed at investigating the performances of certain CO2-philic solvents, including Krytox® GPL 100 PFPE, in physical CO2 absorption. In the paragraph “Conclusions”, it is stated that “The pressure required to dissolve CO2 in PFPE was notably greater than that required for PDMS, PEGDME and PPGDME”. This document does not investigate on the selectivity of the selected solvents towards gases other than CO2.
It is known that the Bunsen coefficient at 20° C. of branched Fomblin® Y25 PFPE for pure N2 (i.e. not in admixture with other gases) is 0.190 and for pure CO2 is 1.30. The Bunsen coefficient at 20° C. of straight Fomblin® Z25 is 0.23 for pure N2 and 1.50 for pure CO2. The Bunsen coefficient is defined as the volume of gas, reduced to 0° C. and to 760 Torr, which is absorbed by the unit volume of solvent (at the temperature of measurement) under a gas pressure of 760 Torr. From the above values it is calculated that, under equilibrium conditions, branched Fomblin® Y25 PFPE has an absorption selectivity CO2/N2 of 6.9 and that linear Fomblin® Z25 has an absorption selectivity CO2/N2 of 6.5. Therefore, these values would lead to expect that branched Fomblin® PFPEs have higher selectivity than linear Fomblin® PFPEs towards CO2 in mixtures of CO2 and N2.
MATSUMOTO, David K., et al. Solubility of Hydrogen and Carbon Monoxide in Selected Nonaqueous Liquids. Ind Eng. Chem. Process Dev. 1985, vol. 24, p. 1297-1300. gives a report of a study aimed at determining the solubility of carbon monoxide in selected non-aqueous liquids, including branched Fomblin® YR PFPE. This fluorinated fluid revealed more effective than the other tested fluids in dissolving gases. On page 1297, left-hand column, this document teaches that gases have greater solubility in highly fluorinated fluids. However, this document does not specifically mention carbon dioxide or other PFPE fluids.
DE 102004053167 A (DEGUSSA [DE]) 4 May 2006 relates to a means for adsorbing gases from gas mixtures, characterized in that it contains a polymer with a branching degree of at least 35% and a molar mass from 500 g/mol to 100,000 g/mol and, optionally, a solvent. The gas can be CO2 and the polymer can be a PFPE.
FR 2923728 A (INST FRANCAIS DU PETROLE [FR]) 22 May 2009 discloses a process for liquefying a gas effluent rich in H2S or CO2 which comprises contacting a gas mixture with a mixture of at least two liquid phases not miscible with one another, at least one of which being an aqueous phase. The phase that is not miscible with water can be a perhalogenated solvent, including a PFPE.
EP 2189416 A (INST FRANCAIS DU PETROLE [FR]) 26 May 2010 discloses a process for the manufacture of hydrogen, said process comprising a step wherein a hydrogen flux containing methane and CO2 are formed and a step of recovery of the methane and CO2 as hydrates by means of a mixture of water and a water-immiscible solvent, including halogenated solvents like hydrofluoroethers (HFEs) and PFPEs.
EP 2201994 A (GEN ELECTRIC [US]) 30 Jun. 2010 relates to an absorbent for CO2 which comprises “a liquid, non aqueous oligomeric material, functionalized with one or more groups that reversibly react with CO2 and/or have a high affinity for CO2”. The functionalized material can be a functionalized PFPE.
There is therefore the need for an alternative method for effectively and selectively removing CO2 from gas streams, said method reducing concerns for environment, health and safety.